Particulate, absorbent, polymeric compositions are capable of absorbing large quantities of liquids such as water and body exudates (e.g., urine) and are further capable of retaining such absorbed liquids under moderate pressures. The absorption characteristics of such polymeric compositions make them especially useful for incorporation into absorbent articles such as diapers. See, for example, U.S. Pat. No. 3,699,103 (Harper et al), issued Jun. 13, 1972, and U.S. Pat. No. 3,770,731 (Harmon), issued Jun. 20, 1972, that disclose the use of particulate, absorbent, polymeric compositions (often referred to as "hydrogels", "superabsorbents", or "hydrocolloid materials") in absorbent articles.
Conventional particulate, absorbent, polymeric compositions, however, have the limitation that the particles are not immobilized and are free to migrate during processing and/or use. Migration of the particles can lead to material handling losses during manufacturing as well as nonhomogeneous incorporation of the particles into structures in which the particles are being used. A more significant problem, though, occurs when these particulate materials migrate during or after swelling in use. Such mobility leads to high resistance to liquid flow through the material due to the lack of stable interparticle capillary or liquid transport channels. This phenomenon is one form of what is commonly referred to as "gel blocking."
One attempt to overcome the performance limitations associated with absorbent particle mobility during use in absorbent articles is incorporation of the particulate, absorbent, polymeric compositions into tissue laminates, i.e. layered absorbent structures. By encapsulating the particles between tissue layers, the overall particle mobility within an absorbent structure is diminished. However, upon liquid contact, the particles within the laminate are often free to move relative to each other resulting in the breakdown of any preexistent interparticle capillary channels.
Another attempted solution is to immobilize the particulate, absorbent, polymeric compositions by the addition of large quantities of liquid polyhydroxy compounds that act as an adhesive to hold the particles together or to a substrate. See, for example, U.S. Pat. No. 4,410,571 (Korpman), issued Oct. 18, 1983. While this approach does limit migration before and, to some extent, during swelling, the particles eventually become detached from each other in the presence of excess liquid, resulting again in the breakdown of any preexisting capillary channels between the particles.
Another attempted solution to overcome the problem of absorbent particle mobility is to produce a superabsorbent film by extrusion of a solution of a linear absorbent polymer and subsequently crosslinking it. See, for example, U.S. Pat. No. 4,861,539 (Allen et al), issued Aug. 29, 1989 (crosslinked with a polyhydroxy compound such as a glycol or glycerol); and U.S. Pat. No. 4,076,673 (Burkholder), issued Feb. 28, 1978 (crosslinked with polyamine-polyamide epichlorohydrin adducts such as Kymene.RTM.). While these superabsorbent films may absorb significant quantities of liquids, they have limited liquid transport properties because they are essentially nonporous, i.e. lack internal capillary channels. Indeed, due to the lack of internal capillary channels, these superabsorbent films are especially prone to gel blocking.
A more recent solution proposed to overcome the problem of absorbent particle mobility is to form these particles into aggregate macrostructures, typically as sheets of bonded absorbent particles. See U.S. Pat. No. 5,102,597 (Roe et al), issued Apr. 7, 1992. These aggregate macrostructures are prepared by initially mixing the absorbent particles with a solution of a nonionic crosslinking agent, water and a hydrophilic organic solvent such as isopropanol. These nonionic crosslinking agents include polyhydric alcohols (e.g., glycerol), polyaziridine compounds (e.g., 2,2-bishydroxymethyl butanoltris[3-(1-aziridine) propionate]), haloepoxy compounds (e.g., epicholorhydrin), polyaldehyde compounds (e.g., glutaraldehyde), polyamine compounds (e.g., ethylene amine), and polyisocyanate compounds (e.g., 2,4-toluene diisocyanate), preferably glycerol. See Column 11, lines 22-54, of Roe et al.
Particulate absorbent polymer compositions of the type used in making these aggregate macrostructures usually contain multiple carboxy groups and are typically derived from polycarboxy compounds such as the polyacrylates. When using glycerol as the crosslinking agent, the hydroxy groups of the glycerol typically react with the carboxy groups of the polymers present in the absorbent particles by an esterification reaction. The crosslinked, ester bond formed by glycerol occurs not only at the surface of the absorbent particles, but also inside particles. This is due to the fact that glycerol is a nonionic, relatively small molecule that can penetrate inside the absorbent particles. The resulting internal crosslinking leads to a lower absorbent capacity for the bonded particles of the aggregate macrostructures.
Moreover, the crosslinking reaction between the hydroxy groups of the glycerol and the carboxy groups of the polymers present in the absorbent particles is relatively slow. Indeed, the glycerol treated absorbent particles are typically cured at 200.degree. C. for 50 minutes. This provides relatively brittle sheets of bonded absorbent particles that are more difficult to handle, especially in making the ultimately desired absorbent structures. Accordingly, these brittle sheets need to be treated with a plasticizer, such as a mixture of water and glycerol, to make them relatively flexible and thus easier to handle in manufacturing absorbent structures.
Accordingly, it would be desirable to be able to make such aggregate macrostructures of bonded absorbent particles using a crosslinking agent that: (1) reacts rapidly with the carboxy groups of the polymer present in the absorbent particles and primarily at the surface thereof to minimize absorbency effects; (2) provides improved absorbency and mechanical (tensile) properties for the aggregate macrostructures; (3) provides flexible sheets of such aggregate macrostructures that can be easily made into absorbent structures used in diapers, adult incontinence pads, sanitary napkins and the like; and (4) does not necessarily require organic solvents such as isopropanol.